1. Field of the Invention
The present invention relates to an optical amplifier for use in a long-distance optical transmission system and an optical network, and in particular, to an optical fiber amplifier.
2. Description of the Related Art
An erbium-doped fiber amplifier (EDFA) amplifies an optical signal of 1.5 .mu.m band, being a low-loss region of an optical fiber, and for this feature, is widely used for an optical communication system. In particular, the EDFA having both a high gain and a low noise figure (NF), has a function of extending a distance between amplifiers in an long-distance optical transmission system, and compensates for a switching loss and a distribution loss in an optical network. Therefore, the EDFA is an essential element in realizing an economical, effective optical communication system.
However, a common EDFA having a single-stage structure can be hardly made such that it has both the high gain and the low noise figure, due to the feature of an erbium-doped fiber (EDF). Therefore, a study has been made of a multi-stage EDFA having both the high gain and the low noise figure.
In an optical communication system, the multi-stage amplifier is made such that a first stage has the low noise figure and the next stage(s) has the high gain and the high output power. Further, in order to increase an efficiency of a pump laser, the multi-stage amplifier includes an optical isolator, a bandpass filter, an optical circulator, a pump reflector and an attenuator between the stages, so as to reduce ASE (Amplified Spontaneous Emission) noise.
However, in the conventional optical fiber amplifier including the bandpass filter to increase the efficiency of the amplifier, a gain bandwidth of the amplifier is limited by a bandwidth of the bandpass filter.
The following patents each discloses features in common with the present invention but do not teach or suggest the specifically recited optical fiber amplifier using a synchronized etalon filter of the present invention: U.S. Pat. No. 5,521,753 to Fake et al., entitled Multi-Stage Fiber Amplifier, U.S. Pat. No. 5,280,383 to Federici et al., entitled Dual-Stage Low Power Optical Amplifier, U.S. Pat. No. 5,566,018 to Lee et al., entitled Apparatus For Adjusting Channel Width Of Multi-Channel Fiber Amplifier Light Source, U.S. Pat. No. 5,808,787 to Meli et al., entitled Amplified Telecommunication System For Wavelength-Division Multiplexing Transmissions, U.S. Pat. No. 5,815,308 to Kim et al., entitled Bidirectional Optical Amplifier, U.S. Pat. No. 5,623,362 to Mitsuda et al., entitled Erbium-Doped Fiber Amplifier And An Optical Fiber Communication System, U.S. Pat. No. 5,295,217 to Marcerou et al., entitled Amplifier Having An Amplifying Optical Fiber, U.S. Pat. No. 5,815,518 to Reed et al., entitled Article Comprising A Cascaded Raman Fiber Laser, U.S. Pat. No. 5,831,754 to Nakano, entitled Optical Amplifier, U.S. Pat. No. 5,838,487 to Nilsson et al., entitled Optical Amplifiers, U.S. Pat. No. 5,778,132 to Csipkes et al., entitled Modular Optical Amplifier And Cassette System, U.S. Pat. No. 5,710,659 to Cline, entitled Low Tilt, High Gain Fiber Amplifier, U.S. Pat. No. 5,664,131 to Sugiya, entitled Light Amplifier, U.S. Pat. No. 5,677,786 to Meli, entitled Device For Reducing The Optical Noise Due To Four Wave Mixing, U.S. Pat. No. 5,701,194 to Meli et al., entitled Amplified Telecommunication System For Wavelength-Division Multiplexing Transmissions Capable Of Limiting Variations In The Output Power, U.S. Pat. No. 5,497,265 to Fontana et al., entitled High-Power Signals Optical Generator For Telecommunication Systems, U.S. Pat. No. 5,260,823 to Payne et al., entitled Erbium-Doped Fiber Amplifier With Shaped Spectral Gain, U.S. Pat. No. 5,550,671 to Simpson et al., entitled Intra-Cavity Optical Four-Wave Mixer And Optical Communications System Using The Same, U.S. Pat. No. 5,808,788 to Park et al., entitled Optical Fiber Amplifier, U.S. Pat. No. 5,434,877 to Chung, entitled Synchronized Etalon Filters, and U.S. Pat. No. 5,440,417 to Chung et al., entitled System For Spectrum-Sliced Fiber Amplifier Light For Multi-Channel Wavelength-Division-Multiplexed Applications.